Replacement of precious metal powder by copper powder for thick film paste hybrid circuit applications is a current trend in the electronic circuit fabrication industry. Copper offers several economic and performance advantages over precious metals. However, copper thick film paste technology is in its infancy, and many improvements still must be made in the process. One of the improvement is reduced cost. One of the ways by which costs may be reduced is to use a smaller diameter copper powder which allows a thinner circuit trace to be fabricated.
One of the better commercial processes for producing such copper particles is by gas atomization. Only a small percentage of the powder produced by atomization is less than about 15 micrometers in maximum particle size. An even smaller percentage is below about 3 micrometers in size. Therefore, yields are low and metal powder costs are high as a result.
Another process for producing copper powders for thick film electronic circuit applications is by chemical or electrical precipitation. While these powders are made with a high yield under 10 micrometers in diameter, the particles are not smooth and spherical. This leads to problems with flowability, packing density, and organic vehicle removal.
U.S. Pat. Nos. 3,909,241 and 3,974,245 to Cheney et al. relates to free flowing powders which are produced by feeding agglomerates through a high temperature plasma reactor to cause at least partial melting of the particles and collecting the particles in a cooling chamber containing a protective gaseous atmosphere where the particles are solidified.
U.S. Pat. 4,264,354 to Cheetham relates to producing spherical dental alloy powders by high frequency induction coil heating followed by cooling in a liquid medium.
In European Patent Application W08402864 published Aug. 2, 1984, there is disclosed a process for making ultra-fine powder by directing a stream of molten droplets at a repellent surface whereby the droplets are broken up and repelled and thereafter solidified as described thereinn. While there is a tendency for spherical particles to be formed after rebounding, it is stated that the molten portion may form elliptical shaped or elongated particles with rounded ends.
U.S. Pat. Nos. 4,711,660 and 4,711,661 relate to spherical particles and process for producing same by mechanically reducing the particles size of the material and high temperature processing followed by rapid solidification. The oxygen content of the spherical particles when the material is reduced in size by the preferred attritor milling is typically greater than about 0.8 % by weight. It is desirable that the oxygen content be lower than this value for better sintering and better mechanical properties, etc.